REPTILES. 347 



tent organ of the male in the kiwer forms (0])lii(lia antl Laccrtilia) is paired. Though, 

 as a rule, reptiles are oviparous;, there are several forms which retain the eggs, until 

 they are hatched, in an enlargement of the ovarian tube. These forms, of which 

 many viperine snakes, as well as the horned toads and some of the ajiodal lizards, are 

 illustrative, are said to be ovoviviparous. 



As to the developmental history of this braucli, that of the birds is foreshadowed. 

 The egg is of considerable size, and often contiins a suiijily of food for the growing 

 embryo. By a folding together of the ventral walls of the body, the embryo, 

 though at first lying prone u|)on the egg, is finally only connected with it by 

 a small jieduuc'le, the umbilicus, through which is drawn the nourishment of the food 



yolk. 



The amnion is a thin membrane enclosing the embryo, which floats in liquid. The 

 allantois is an organ of embryonic life, and jierforms the office of a resi)iratory sac. It 

 is aj)i)ended to the posterior ])ortion of the alimentary tract, and is oidy met with in 

 those animals which are unprovided with gills, and which do not, on leaving the egg, 

 j)ass through metamorphoses. These two organs, the amnion and allantois, first 

 appear in the rejitiles. 



Having now, in a cursory way, examined the general points of structure presented 

 bv the various members of the class, an inquiry into its relations can be profitably 

 made ; it is here that the real value of paleontology presents itself. By considering 

 the fossil forms, the reptiles are seen to pass imperceptibly into the birds, and the birds 

 are found to reach over, as it were, and greet the reptiles. The dinosaurs were rejv 

 tiles having the pelvis, hind limbs, and feet strongly resembling those of the ostrich, 

 and some of the bones of the body were supplied with air-cavities. Many were biped, 

 the anterior limbs being extremely small. That they were provided with teeth does 

 not argue their non-avian affinities, for the lower birds, like ITesperoniis, were well 

 supplied with these organs of prehension. The most remarkable avian peculiarities 

 are presented by members of the highest order, Pterosauria. These flying reptiles had 

 the bones of the fore limbs resembling, to no little extent, those of birds ; the neck and 

 head were long ; the jaws were sometimes toothless, and encased in horn ; the tail was 

 short, and the shoulder-girdle, keeled sternum, and hollow bones carried the resem- 

 blance still further. As a result of these resemblances, together with those advanced 

 by the anatomist and erabryologist, the birds have been united with the reptiles into 

 a single group, the Sauropsida, the propriety of which arrangement is daily more and 

 more evident. 



Although the snakes and a few lizards extend well up into the temperate regions, 

 which is the true home of the turtles, by far the greater number are found in the 

 torrid zone, from which the Crocodilia do not ordinarily wander. As reptiles are 

 animals illy adapted for migration, they endure the cold of winter by passing into 

 a tor])id sleep — hibernation, and the enormous heat of the tropical sun by a some- 

 what similar summer sleep. 



Reptiles first made their geological appearance during the carboniferous age, 

 and abounded during the mesozoic, when they were rulers of the air, land, and 

 sea. The classification now generally adopted divides the class Reptilia into eleven 

 orders. 



